The aim of this research is to delineate the effects of drugs such as hallucinogens (LSD) and narcotics (morphine) on perceptual behavior and to determine the extent to which such effects are due to drug-induced changes in sensitivity (sensory capacity) or in judgement (response bias). We hope to achieve our goal by establishing standardized and well-structured test situations involving successive, 2-choice, discrete-trial, discrimination training, by systematically varying sensory as well as non-sensory parameters in these situations, and by measuring behavior over a range of drug dosages. Two species will be used--rats and pigeons. Discrimination behavior will be analyzed within the framework of signal detection theory by using procedures which permit performance to be determined differentially by the physical parameters of auditory, visual and tactile (shock) stimuli and non-stimulus parameters such as the relative probabilities of reinforcing different choice behaviors.